Aqueous slurry process for drying of solvent wet double base smokeless powder

ABSTRACT

A process for rapid drying of solvent-wet double-base smokeless powder is provided in which most of the solvent entrapped in the double-base powder particles manufactured by the conventional solvent process is extracted from the powder into the aqueous phase of an aqueous slurry of the powder. The solvent-extracted smokeless powder is then dried employing rapid-drying techniques with no substantial loss of energetic plasticizer.

United States Patent Inventors John O. Bonyata AQUEOUS SLURRY PROCESSFOR DRYING OF SOLVENT WET DOUBLE BASE SMOKELESS POWDER 9 Claims, NoDrawings Int. Cl C06b 21/02 Field of Search 264/3;

[56] References Cited UNITED STATES PATENTS 438,816 10/1890 Lundholm eta1 149/97 2,860,042 11/1958 McDonnell et al. 149/97 3,037,891 6/1962Maag 149/97 Primary Examiner-Benjamin R. Padgett AssistantExaminer-Stephen J. Lechert, Jr. Attorney-S. Grant Stewart ABSTRACT: Aprocess for rapid drying of solvent-wet double-base smokeless powder isprovided in which most of the solvent entrapped in the double-basepowder particles manufactured by the conventional solvent process isextracted from the powder into the aqueous phase of an aqueous slurry ofthe powder. The solvent-extracted smokeless powder is then driedemploying rapid-drying techniques with no substantial loss of energeticplasticizer.

AQUEOUS SLURRY PROCESS FOR DRYING OF SOLVENT WET DOUBLE BASE SMOKELESSPOWDER This invention relates to an aqueous slurry process for highspeeddrying of solvent-wet double-base smokeless powder.

The time honored conventional process for manufacture of smokelesspowder involves as a first step the granulation of nitrocellulose. Inthis process water wet nitrocellulose is dehydrated with ethyl alcoholin a blocking press. The resulting dehydrated nitrocellulose is brokenup in a block breaker and subsequently masticated in a large mixer bythe addition of ethyl alcohol and acetone to form a homogeneous stiffdough. Nitroglycerin or other energetic plasticizer is admixed with thedough during the mixing process to form a double base dough. The stiffdough is blocked in large hydraulic presses and subsequently is extrudedat high pressures into strands of colloided double-base powder. Thestrands of powder are then cut to a desired length and dried in a dryhouse.

In the conventional drying process the cut smokeless powder granulescontaining from about 8 percent to about percent solvent, based on theweight of the wet smokeless powder are taken to a dry house, placed intrays and dried in a fixed bed. Fixed bed drying of the smokeless powdergranules takes place by a combination of conduction and convection andpermits the processing solvents employed generally alcohol and acetone,to diffuse out of the powder granules at a slow rate carrying along onlyvery small quantities of nitroglycerin. As the solvents reach thesurface of the smokeless powder g ranules they pass from the liquid intothe vapor phase. In this drying process the transfer of heat requiredfor rapid evaporation of the solvents from the smokeless powder isrestricted because of concentration gradients of solvent vaporssurrounding the smokeless powder particles and because a surface film isformed on the smokeless powder particles by the presence of the solvent.From the top of the powder bed to the center of the powder bed which isformed on the drying tray there is a gradation in solvent and watervapor. For this reason smokeless powder granules at the top of the beddry first and those granules in the center of the bed dry last.

If an attempt is made to increase the drying rate of the powder bythinning out the bed of smokeless powder being dried or by forcing airthrough the bed as in a fluidized bed drying system, the concentrationgradient of solvent and moisture vapor through the bed is removed. Whenthis concentration gradient is removed the solvent does not fractionateout of the powder bed and nitroglycerin which is soluble in the solventsmigrates with the solvents to the surface of the smokeless powdergranules and is stripped therefrom. As the concentration ofnitroglycerin in smokeless powder increases, the amount of nitroglycerinthat dissolves in the solvents and is stripped from the powder employingrapid-drying techniques will also increase.

The conventional fixed bed-drying process for double base smokelesspowder described is a low-drying rate process because of the necessityto maintain solvent gradients through the bed of powder in order toeliminate stripping of nitroglycerin from the powder. Typical dry houseconditions for drying doublebase smokeless powder granules in a fixedbed are temperatures of from about 125 F. to about 160 F., drying timesfrom about 3 days to about 5 days, and powder bed thickness of fromabout 1.5 inches to about 3.0 inches.

In accordance with the method of this invention solvent-wet double-basesmokeless powder is dried by a process comprising forming an aqueousslurry of solvent-wet double-base powder particles and water, the waterhaving dissolved therein an equilibrium concentration of energeticplasticizer. The energetic plasticizer dissolved in the water is thesame plasticizer employed in manufacture of the smokeless powder. Theslurry which is formed has a continuous phase comprising water, andenergetic plasticizer, and a dispersed phase comprising smokeless powderparticles. The slurry is agitated continuously until it is comprised ofa continuous phase of water,

energetic plasticizer, and solvent(s), and a dispersed phase comprisingsolvent-extracted smokeless powder particles. The resultingsolvent-extracted smokeless powder particles are then separated from theaqueous slurry. The separated smokeless powder particles are thenrapidly dried to remove residual solvent and water.

In the process described. processing solvents are rapidly extracted fromthe solvent-wet smokeless powder granules into the water phase of theslurry until the solvent extracted powder contains less than about 1.5percent by weight of solvents based on the weight of the extractedpowder. The water phase of the slurry also has dissolved therein anequilibrium concentration of energetic plasticizer. The term equilibriumconcentration of energetic plasticizer as used herein is defined as aconcentration of energetic plasticizer in the aqueous slurry at whichthere is substantially no tendency for the energetic plasticizer to beeither absorbed into or extracted from the smokeless powder granules.The solvent extracted smokeless powder particles are separated from theslurry and rapidly dried. Due to the greatly reduced concentration ofsolvent in the smokeless power particles, substantially no nitroglycerinis stripped from the particles during rapid drying.

The following examples more fully illustrate this invention. All partsand percentages are by weight unless otherwise specified.

The following example illustrates the loss of nitroglycerin fromsmokeless powder when employing rapid drying techniques. Comparison ismade with the conventional dry house" dry procedure.

EXAMPLE 1 A conventionally granulated solvent wet smokeless powder isextruded through a 0.065 inch I.D. (inside diameter) X 0.020 inch O.D.(outside diameter) die and cut at 300 cuts per lineal inch. The powderis divided into two equal portions. One-half of the smokeless powder isdried in a conventional manner in a dry house for 3 days at about l35 F.The other half of the smokeless powder is dried very rapidly in afluidized bed by forcing air at F. through the bed of powder. Theinitial nitroglycerin content of the powder is 20 percent. The resultsof drying these powders are listed in table 1 below.

As can be seen from table I, the conventionally dried smokeless powderrequires about 46 hours of drying to bring the moisture and volatilecontent of the powder to an acceptable level (0.50 percent or less).There is no measurable nitroglycerin loss. The powder dried by the rapiddrying process requires about 5 hours of drying to bring the moistureand volatile content of the powder to the acceptable level. Over 5percent loss of nitroglycerin from the powder results. based on theoriginal nitroglycerin content of the powder.

Examples 2-5 which follow illustrate the rapid drying process of thisinvention for double base smokeless powder.

EXAMPLES 2-5 A batch of conventionally manufactured smokeless powder isextruded through 0.038 inch die and cut at 250 cuts per lineal inch. Theresulting green powder containing about 12 percent by weight solventbased on the weight of the solventwet powder is divided into four lots.The first lot, example 2, is dried in a conventional manner for a periodof 72 hours at a temperature of 130 F. using standard dry housefacilities. The second lot, example 3, is dried over a period of 72hours at a temperature of 130 F. in an experimental dry house whereineach drying tray is placed on a separate rack.

The remaining two lots of conventionally granulated powder are placed ina vessel and jointly slurried with water to form a slurry comprising adispersed phase of smokeless powder particles and a continuous phase ofwater. The resultant slurry contains 12 percent by weight of smokelesspowder and 88 percent by weight of water based on the weight of theslurry. The water has dissolved therein 0. l percent by weight ofnitroglycerin which is an equilibrium concentration of nitroglycerin.The slurry is agitated to maintain the dispersion of the smokelesspowder in the slurry and temperature of the slurry is increased fromambient temperature 68 F. to 175 F. This temperature is maintained forabout 5 minutes. Colloidal graphite (type GPW, an aqueous pastecontaining 22 percent solids, sold by Graphite Products Corporation) is.

added to the agitated slurry and the slurry maintained at the 175 F.temperature for an additional ten minutes. The slurry is then cooled andcentrifuged.

The resulting glazed smokeless powder contains approximately 15 percentby weight of moisture and processing solvents. The processing solventsare about 1 percent by weight based on the weight of the wet powder.This powder is divided into two lots, 3 and 4. Lot 3 (example 4) isdried in a dry house under the same conditions as example 3. From lot4(example 5) two one-half pound samples are taken and dried in a smallfluid bed type dryer operating at 150' 5 F. The drying time, percentmoisture, and percent nitroglycerin, in the dried smokeless powderparticles prepared in the foregoing examples is set forth in table 2below.

TABLE 2.MOISTURE LEVEL AND NITROGLYCERIN LOSS ON FOUR DRYING TECHNIQUESminutes 0. 50 38:84

The foregoing examples illustrate the reduction in time in dryingsolvent-wet smokeless powder in accordance with the process of thisinvention (example 5). Substantially no nitroglycerin loss results fromthe process of this invention. Consequently, the process is consideredas safe as the conventional dry house process for drying smokelesspowder.

in the drying process heretofore described it is generally preferred toprepare an aqueous slurry comprised of from about'70 percent to about 92percent by weight of water, and

from about 30 percent to about 8 percent by weight of smokeless powderparticles. lf more than 92 percent water is employed, equipment size andprocessing time increase to make processing unwieldy. If less than about70 percent water is employed the extraction efficiency is reduced.

The water employed in preparation of the aqueous slurry has dissolvedtherein an equilibrium concentration of the same energetic plasticizeremployed in manufacture of the solvent wet smokeless powder. Energeticplasticizer is readily dissolved in water to form the equilibriumconcentration. Recycle water from previous cycles can be advantageouslyemployed. The presence of the plasticizer in the water is necessary tomaintain a plasticizer-water-nitrocelIulose equilibrium. When operatingwithin the water-smokeless powder slurry concentration range heretoforedescribed, this equilibrium concentration of energetic plasticizer isfrom about 0.1 to about 0.3 percent by weight based on the weight of thewater. If the energetic plasticizer content in the water is in excess ofabout 0.3 percent, the equilibrium is shifted in favor of driving theenergetic plasticizer into the smokeless powder particle. if theenergetic plasticizer concentration falls below about 0.1. percent thenmigration of energetic plasticizer into the water is favored. Neithercondition is desirable since it is important to closely control thecomposition of the smokeless powder which in turn controls burning ratecharacteristics of the powder.

After forming the aqueous slurry of this invention, extrac tion ofprocessing solvents from the solvent-wet smokeless powder into thecontinuous water phase of the slurry begins. The rate of extraction ofsolvents from the powder is increased with increasing slurrytemperature. Preferred temperature ranges for extraction are from aboutto about 190 F. When operated in the preferred extraction temperatureranges, the time required for solvent extraction will vary from about 5minutes to about 15 minutes. The resultant smokeless powder particlesare separated from the slurry by a suitable means such as filtration ordecantation and contain from about 0.5 to about l.5 percent by weight ofresidual solvent based on the weight of the extracted powder. This lowamount of solvent can be safely removed in a high-speed forced air fluidbed dryer without causing any substantial loss of nitroglycerin from thepowder during drying as illustrated in example 5.

The extraction rate of the solvents from the smokeless powder into thecontinuous aqueous phase of the slurry can be increased by continuouslyseparating extracted solvents from the slurry. A simple and effectivemanner of continuously separating these solvents from the slurry is bypassing an inert gas or air through the agitated and heated slurry. Thegas passing through the slurry will aid vaporization of the solventsfrom the slurry. The resulting solvent rich gaseous mixture can then bepassed through a condenser to recover the solvents. Other methods ofseparating solvent from the slurry such as continuously withdrawing aportion of the slurry, stripping solvent therefrom, and recycling of thewater-plasticizer phase to the slurry are acceptable.

In all of the processing steps heretofore described, the agitation mustbe sufficient to maintain the smokeless powder par ticles dispersedthroughout the water phase of the slurry. This type of agitation isreadily supplied by simple paddle or propeller type mixers.

Prior to separating the extracted smokeless powder particles from theaqueous slurry heretofore described, colloidal graphite can be admixedwith the slurry. The function of the colloidal graphite is to provide anelectrically conductive coating on the particles to reduce electrostaticpotential on the particle permitting removal of residual solvent insafety and to prevent agglomeration of the smokeless powder particles.When drying large smokeless powder particles such as cannon powder inwhich the powder is about 0.15 inches in diameter or greater, thecolloidal graphite can be omitted since no agglomeration problem exists.Colloidal graphite applied in the slurry coats the surface of thesmokeless powder particles and firmly adheres thereto when the smokelesspowder particles are separated from the aqueous slurry. The amount ofcolloidal graphite employed on the powder is generally from about 0.1 toabout 2.0 percent by weight based on the weight of the smokeless powderparticles (dry).

The temperature of the slurry required for graphite glazing is 150 F. orhigher. Preferred glazing temperatures are from about l70-l90 F. Thetime required to graphite glaze the smokeless powder particles whenoperating in the preferred temperature range is from about 5 minutes toabout 30 minutes.

Following the graphite glazing step the heating of the slurry is stoppedand the solvent-extracted and graphite-coated smokeless powder particlesare separated from the aqueous slurry and are dried.

Rapid removal of residual solvent from the extracted double base powdercan be conducted in a fluidized-bed-type dryer. Operating conditions forfluidized bed drying will vary, depending upon powder bulk density, anddegree of dryness desired. These conditions include gas flow rate, gastemperature, depth of smokeless powder, and residence time. Typical gastemperatures are from about 130 to about 160 F. Gas flow rate must becontrolled to avoid excessive turbulence of particles which can causedegradation of the particle geometry and result in an unsafe operatingcondition. It is well within those skilled in the art of drying toadjust gas flow rates and temperature 5 to rapidly dry the solventextracted smokeless powder of this invention.

While the foregoing process description has made specific reference topreventing loss of nitroglycerin as the energetic plasticizer in theaqueous slurry drying process of this invention it is not so limited.Other energetic plasticizers which can be employed in the double-basepowder and dried by the process of this invention include the nitrateesters of polyhydric alcohols such as diethylene glycol dinitrate,ethylene glycol dinitrate and the like. Double-base powders oftencontain mixtures of energetic plasticizers such as those described andnonenergetic plasticizers such as triacetin, dibutyl phthalate,tricresyl phosphate, and the like. When a mixture of plasticizers hasbeen prepared, the water portion of the slurry can contain in additionto the equilibrium concentration of energetic plasticizer, anequilibrium concentration of nonenergetic plasticizers. Theseconcentrations can be readily determined by simple mixing of a smokelesspowder aqueous slurry, and sampling the water phase of the slurry. Whenthe plasticizer level remains constant, the equilibrium concentrationfor the smokeless powder employed at the particular ratio of smokelesspowder to water in the slurry, is determined. When a recycle system isemployed, the equilibrium concentration of energetic plasticizer in thecontinuous aqueous phase is rapidly attained, and this concentrationwill remain constant in subsequent cycles in the process.

What I claim and desire to protect by Letters patent is:

1. An aqueous slurry process for rapid drying of solvent-wet double-basesmokeless powder particles said aqueous slurry drying processcomprising:

a. forming an aqueous slurry of solvent-wet doublebase smokeless powderparticles and water, said water having dissolved therein an equilibriumconcentration of energetic plasticizer, said energetic plasticizerdissolved in the water being a plasticizer of the same compositionemployed in manufacture of the solvent-wet double-base smokeless powderparticles, said aqueous slurry initially having a continuous phasecomprising water and dissolved energetic plasticizer, and a dispersedphase comprising solvent-wet smokeless powder particles, said slurrybeing agitated to maintain the smokeless powder particles in thedispersed phase until the slurry is comprised of a continuous phasecomprising water, energetic plasticizer and solvent, and a dispersedphase comprising solvent-extracted smokeless powder particles,

b. separating the resulting solvent-extracted smokeless powder articlesfrom the aqueous slurry, and c. drying t e resultant powder to removeresidual solvent and water.

2. The process of claim 1 in which the energetic plasticizer isnitroglycerin.

3. The process of claim 2 in which the aqueous slurry of step (a) isheated to a temperature of from about to about 190 F., said aqueousslurry comprising from about 70 to about 92 percent by weight of waterand from about 30 to about 8 percent by weight of solvent-wet doublebase smokeless powder particles, said water containing from about 0.1 toabout 0.3 percent of nitroglycerin based on the weight of the water.

4. The process of claim 3 wherein the aqueous slurry is agitated untilthe solvent extracted smokeless powder contains less than about 1.5percent solvent based on the weight of the solvent extracted smokelesspowder.

5. The process of claim 4 in which the solvents extracted from thesolvent-wet double-base smokeless powder particles into the continuousphase of the aqueous slurry are continuously separated therefrom.

6. The process of claim 5 in which the solvents extracted from thesolvent-wet double-base smokeless powder particles into the continuousphase of the aqueous slurry are separated from the aqueous slurry byblowing inert gas or air through the slurry.

7. The process of claim 3 in which the aqueous slurry of step (a) isheated to a temperature of at least F. and colloidal graphite isdispersed in the heated aqueous slurry, the temperature of the heatedslurry being maintained at at least l50 F. until substantially all ofthe colloidal graphite adheres to the surfaces of the smokeless powderparticles.

8. The process of claim 7 wherein from about 0.1 to about 2 percentcolloidal graphite is dispersed in the slurry based on the weight of thesmokeless powder.

9. The process of claim 7 in which the separated, solvent-ex' tractedand graphite glazed smokeless powder particles are dried in a fluidizedbed drier.

2. The process of claim 1 in which the energetic plasticizer isnitroglycerin.
 3. The process of claim 2 in which the aqueous slurry ofstep (a) is heated to a temperature of from about 80* to about 190* F.,said aqueous slurry comprising from about 70 to about 92 percent byweight of water and from about 30 to about 8 percent by weight ofsolvent-wet double base smokeless powder particles, said watercontaining from about 0.1 to about 0.3 percent of nitroglycerin based onthe weight of the water.
 4. The process of claim 3 wherein the aqueousslurry is agitated until the solvent extracted smokeless powder containsless than about 1.5 percent solvent based on the weight of the solventextracted smokeless powder.
 5. The process of claim 4 in which thesolvents extracted from the solvent-wet double-base smokeless powderparticles into the continuous phase of the aqueous slurry arecontinuously separated therefrom.
 6. The process of claim 5 in which thesolvents extracted from the solvent-wet double-base smokeless powderparticles into the continuous phase of the aqueous slurry are separatedfrom the aqueous slurry by blowing inert gas or air through the slurry.7. The process of claim 3 in which the aqueous slurry of step (a) isheated to a temperature of at least 150* F. and colloidal graphite isdispersed in the heated aqueous slurry, the temperature of the heatedslurry being maintained at at least 150* F. until substantially all ofthe colloidal graphite adheres to the surfaces of the smokeless powderparticles.
 8. The process of claim 7 wherein from about 0.1 to about 2percent colloidal graphite is dispersed in the slurry based on theweight of the smokeless powder.
 9. The process of claim 7 in which theseparated, solvent-extracted and graphite glazed smokeless powderparticles are dried in a fluidized bed drier.